1.Field of the Invention
The present invention relates to a sensing unit for a position detecting apparatus using electromagnetic actions, and its manufacturing method.
2. Description of the Related Art
A position detecting apparatus for detecting positions designated with a position indicator has been introduced in the past, wherein position detection is based on electromagnetic actions working between a sensing unit having a plurality of sense wires and a position indicator having at least a coil or a magnetic substance.
FIG. 2 is a top view of an exemplary sensing unit for the foregoing conventional position detecting apparatus, or in this example, an apparatus for detecting x- and y-coordinate values of a position designated with a position indictor. In FIG. 2, 1-1 to 1-8 denote x-axis sense wires. 2-1 to 2-8 denote y-axis sense wires. 3 is an insulating substrate.
The x-axis sense wires 1-1 to 1-8 are one-turn loop coils and set in array to overlap one after another in the x-axis direction. The y-axis sense wires 2-1 to 2-8 are one-turn loop coils and set in array to overlap one another in the y-axis direction. The sense wires 1-1 to 1-8 and 2-1 to 2-8 are connected to electronic components (not shown) mounted on the insulating substrate 3 via lines 4-1 to 4-8 and 5-1 to 5-8 for sending or fetching signals to or from the sense wires. The sense wires 1-1 to 1-8 and 2-1 to 2-8, and the lines 4-1 to 4-8 and 5-1 to 5-8 are formed by combining two plies of conductor patterns produced on both sides of the insulating substrate 3 (the patterns are indicated with solid lines and dashed lines in FIG. 2).
The aforesaid sensing unit of prior art is provided with a shielding plate for preventing influence of external electromagnetic waves and reducing external leakage of an electromagnetic wave. The shielding plate is installed on one side of the sensing unit (on the side which is not used to designate positions with a position indicator).
FIG. 3 is a side sectional view of a conventional sensing unit having the foregoing shielding plate. In FIG. 3, 3 denotes an insulating substrate. 6 and 7 are conductor patterns. 8 and 9 are lands. 10 and 11 de:note overcoat. 12 denotes a through hole. 13 is a shielding plate. 14 is a spacer. 15 and 16 denote double-sided adhesive tape.
Conductor patterns 6 and 7 form sense wires 1-1 to 1-8 and 2-1 to 2-8, and lines 4-1 to 4-8 and 5-1 to 5-8, which are produced by etching copper foils adhered on both sides of an insulating substrate 3 formed with a glass epoxy substrate. Parts of the conductor patterns 6 and 7 are bared to drill through holes for mounting an electronic component or allowing the conductor patterns 6 and 7 to conduct, thus forming lands 8 and 9. The insulating substrate 3 except the lands 8 and 9 and the conductor patterns 6 and 7 are coated with overcoat 10 and 11 made from epoxy resin or other insulating material by means of screen printing. After overcoat processing, a through hole 12 is produced through numerically-controlled drilling, and solder-plating (17).
A shielding plate 13 is made of silicon steel or stainless alloy, and adhered to one side of an insulating substrate 3, or in this example, a side covered with a conductor pattern 7 and overcoat 11 using double-sided adhesive tape 16 with double-sided adhesive tape 15 and a spacer 14 interposed. The spacer 14 is interposed to prevent the shielding plate 13 from approaching the conductor pattern 7 too closely and thus adversely affecting the electromagnetic action working between the conductor pattern 7 and a position indicator (not shown).
However, the aforesaid sensing unit has problems mentioned below. First, a through bole 12 must be drilled to allow conductor patterns 6 and 7 to conduct. However, as described previously, numerically-controlled drilling must be done to produce the through hole 12. As the through hole 12 grows in number, more time is required, yield deteriorates, and cost increases. Secondarily, double-sided adhesive tape is employed to mount a shielding plate 13, especially, a spacer on an insulating substrate 3, because when adhesive is directly applied, the adhesive may come out of the opposite side through the through hole 12. Then, the insulating substrate 3 having conductor patterns, shielding plate 13, and spacer 14 are molded to have the same dimensions in a well-known dicing process. After that, the spacer is mounted on the insulating substrate 3 by an experienced operator. This results in poor productivity and high cost. The third problem is that the conductor pattern 6 forming mainly x-axis sense wires 1-1 to 1-8 is separated from the conductor pattern 7 forming mainly y-axis sense wires 2-1 to 2-8 by the thickness of the insulating substrate 3 (normally, about 1 mm). This creates a difference equivalent to the thickness between the distance of the conductor pattern 6 from a position indicator and the distance of the conductor pattern 7 from the position indicator. The difference between the distances results in a difference between the electromagnetic actions in the x-axis and y-axis directions. This deteriorates reading efficiency for assuring position detecting precision. The fourth problem is that the spacer 14, which does not contribute to position detection at all, must be installed. Therefore, the sensing unit becomes thicker by the thickness of the spacer 14.